272 ZOOTOMV. 



sphenoid, below with the alisphenoid, and behind with 

 the descending process of the parietal. From its outer 

 face is given off the zygomatic process, a strong bar 

 of bone which passes first outwards, then downwards, and 

 lastly forwards, and which bears on the under surface of 

 its outwardly directed portion an articular surface the 

 glenoid cavity for the lower jaw. From the posterior 

 edge of the squamosal a slender process is given off, which 

 passes backwards and downwards against the surface of the 

 periotic ( 56). 



54. The tympanic, a bone constituting the osseous 

 part of the auditory meatus. It consists of a tubular 

 portion above, and of a swollen portion below, which 

 projects on the under surface of the skull and articu- 

 lates with the lateral edge of the basioccipital : this last 

 is the bulla tympani. Both tubular and bulbous 

 portions are incomplete on the inner face, a vacuity being 

 left which is only filled up when the tympanic bone is 

 applied against the periotic. At the junction of the 

 tubular and bulbous portions the tympanic is produced on 

 its inner surface into a curved flattened ridge, to which the 

 tympanic membrane is attached ( 346). 



55. On the posterior border of the tympanic, at the 

 junction of its tubular and bulbous portions is a shallow 

 groove : when the tympanic is in place this notch is con- 

 verted by the periotic into a canal, the stylo-mastoid 

 foramen, which transmits the seventh nerve. A notch in 

 the postero-inferior region of the bone is also converted 

 into a canal by the juxtaposition of the periotic, forming the 

 bony portion of the Eustachian tube ( 351). 



56. The periotic : a very irregular bone, articulating 

 with the exoccipital behind, the supraoccipital above, the 

 descending process of the parietal in front, and the basi- 



